The present invention, in some embodiments thereof, relates to optics and, more particularly, but not exclusively, to intraocular and contact lenses.
The human eye is a complex anatomical device, which facilitates interpretation of shapes, colors and dimensions of objects by processing the light they reflect or emit. Similarly to a camera, the eye is able to refract light and produce a focused image that can stimulate neural responses and provide the ability to see.
For the purpose of providing a self-contained document, following is a description of the principle of operation of the mammalian eye, in general, and of the cornea in particular. The iris regulates the amount of light admitted to the interior of the eye, the cornea and the lens focus the light rays from an object being viewed onto the retina which transmits the image of the object to the brain via the optic nerve. About 75% of the focusing is provided by the cornea, with the other 25% provided by the crystalline lens which may acquire variable focal lengths.
The cornea is the most anterior structure of the eye. Since it has to be transparent to allow light to enter the eye, there are no blood vessels in the cornea. The cornea is composed of collagen fibers packed together in an organized pattern, thereby providing the cornea its light transparent nature. The cornea has the highest concentration of nerve endings in the entire body, thus making it extremely sensitive to any kind of trauma.
The front view of the cornea is of an aspheric shape, where the vertical dimension is smaller than the horizontal dimension by about 1-2%. The anterior is typically about 11.7 mm in diameter.
The quality of vision depends on many factors including the size and shape of the eye, and the transparency of the cornea and lens. When age or disease causes the lens to become less transparent, vision deteriorates because of the diminished light which can be transmitted to the retina. This deficiency in the lens of the eye is medically known as a cataract. An accepted treatment for this condition is surgerical removal of the lens and replacement of the lens function by an intraocular lens (IOL).
Over the years, numerous types of IOLs have been developed for correcting vision deficiencies. Generally, such lenses operate accordion to one two basic optical principles: refraction and diffraction.
A typical IOL is manufactured from polymethyl methacrylate, has a diameter of about 5-7 mm, and is supported in the eye by the spring force of flexible loops called haptics. Other materials are also used, and there are a variety of lens style and haptic designs.
Multifocal lens has more than one point of focus. A bifocal, which is a type of multifocal, has two points of focus, one at distance and the other at near. In multifocal IOL the aim is to increase the range of distinct vision and hence to reduce the dependence on additional spectacle corrections. Rigid lenses that have two or more optical powers are used to divide the incident light between axially separated images. Overall image quality is affected by the number of lens powers, and the image quality of the focused component itself.
One type of multifocal IOL is diffractive multifocal IOL. A pair of diffraction orders is used to provide two lens powers simultaneously by using rigid implant. One power is used for distance vision and the other power is used for near vision. In both cases defocused light is also incident on the retina, but the human visual system is tolerant of contrast-related image variations and this does not appear to be a problem for most patients. The diffractive design utilizes the full aperture and is tolerant of pupil size variations and modest decentration.
Generally, a diffractive lens consists of any number of annular lens zones of equal area. Between adjacent zones optical steps are provided with associated path length differences which usually are absolutely smaller than a design wavelength. The area or size of the zones determines the separation between the diffractive powers of the lens; this separation increases with decreasing zone area. The optical path difference determines the relative peak intensities of the various diffractive powers. For example, when the optical path difference equals half the wavelength there are two principal diffractive powers, the zeroth and the first order diffractive power. For absolute path differences which are smaller than half the wavelength, the zeroth order power is dominant, while for optical path differences which are of order of one wavelength the first diffractive order power is dominant.
Also known are lenses which are based on refractive principles. Such refractive lenses typically include concentric zones of differing power.
U.S. Pat. No. 4,338,005 discloses a multiple focal power optical device which includes a plurality of alternating annular concentric zones. At least some of the zones include focal power means for directing incident parallel light to a first focal point, and at least some of the zones include focal power means for directing incident parallel light to a second focal point. The radius of the nth zone is proportional to the square root of n, and the radius of the first zone is proportional to the square root of the wavelength under consideration.
U.S. Pat. No. 5,089,023 discloses an intraocular optical implant which includes a refractive/diffractive lens having an anterior surface and a posterior surface and a generally anterior-posterior optical axis. At least one of the anterior and posterior surfaces of the lens has a diffractive lens profile covering about half the effective lens area of the lens.
U.S. Pat. No. 5,699,142 discloses a diffractive multifocal ophthalmic lens having an apodization zone that gradually shifts the energy balance from the near focus to the distance focus over a substantial portion of the lens so that the outer region of the lens directs all of its energy to the distance focus.
U.S. Pat. No. 6,536,899 disclose a multifocal lens including a plurality of annular zones. Each annular zone is divided into two annular sub-zones such that the refractive powers within the sub-zones exhibit at least two diffractive powers and at least one of the diffractive powers substantially coincides with the average refractive power of each annular zone.
Additional background art includes U.S. Pat. Nos. 4,881,805, 5,344,47, 7,377,641, 4,162,122, 4,210391, 4,338,005, 4,340,283, 4,995,714, 4,995,715, 4,881,804, 4,881,805, 5,017,000, 5,054.905, 5,056,908, 5,120,120, 5,121,979, 5,121,980, 5,144,483, 5,117,306, 5,076,684, 5,116,111, 5,129,718, 4,637,697, 4,641,934 and 4,655,565, and European Patent No. 1194797B1,